1. Field of the Invention
This invention relates to an illuminating device, and a polarizing illuminating device and a projector using them.
2. Related Background Art
FIG. 1 of the accompanying drawings shows the construction of the essential portions of a projector according to the prior art.
This projector comprises a light source 10511 comprising a halogen lamp, a metal halide lamp or the like emitting non-polarized light, a reflecting mirror 10512 for reflecting part of the non-polarized light emitted from the light source 10511, a heat ray cutting filter 10513 for absorbing or reflecting the heat rays of the non-polarized light incident directly from the light source 10511 or through the reflecting mirror 10512, a condenser lens 10514 for converting the non-polarized light having its heat rays removed therefrom into non-polarized parallel light, a polarizing plate 10515 for converting the non-polarized parallel light into linearly polarized light, a liquid crystal light valve 10517 which is an image generator for modulating the linearly polarized light in conformity with a video signal to thereby generate an image, a polarizing plate 10518 for transmitting therethrough only the component in the direction of the transmission axis of the linearly polarized light modulated by the liquid crystal light valve 10517, and a projection lens 10520 which is a projection optical system for projecting the linearly polarized light transmitted through the polarizing plate 10518 onto a screen (not shown) and projecting said image.
FIG. 2 of the accompanying drawings shows the construction of the essential portions of another projector according to the prior art.
This projector has two polarizing beam splitters 10516 and 10519, instead of the two polarizing plates 10515 and 10518 of the projector shown in FIG. 1, disposed forwardly and rearwardly of the liquid crystal light valve 10517.
In the projectors shown in FIGS. 1 and 2, of the non-polarized light emitted from the light source 10511, only the linearly polarized light transmitted through the polarizing plate 10515 or the polarizing beam splitter 10516 is utilized as illuminating light for the liquid crystal light valve 10517, and this leads to a problem that the linearly polarized light not transmitted through the polarizing plate 10515 or the polarizing beam splitter 10516 is lost and the utilization efficiency of the light becomes 50% or less. FIG. 3 of the accompanying drawings shows a projector which eliminates such problem.
In this projector, non-polarized parallel light emerging from the condenser lens 10514 enters a polarizing beam splitter 10521, and P-polarized light L.sub.P is intactly transmitted through the acting surface 10521a of the polarizing beam splitter 10521 (deposited film formed on the slopes of two rectangular prisms which are adhesively secured to each other) and S-polarized light L.sub.S is upwardly reflected at a right angle as viewed in FIG. 3 and enters a total reflection prism 10522. The S-polarized light L.sub.S is rightwardly reflected at a right angle by the total reflection prism 10522 as viewed in FIG. 3, whereby it emerges from the total reflection prism 10522 in the same direction as the P-polarized light L.sub.P transmitted through the polarizing beam splitter 10521. Here, the S-polarized light L.sub.S refers to linearly polarized light having a plane of polarization parallel to the acting surface 10521a of the polarizing beam splitter 10521, and the P-polarized light L.sub.P refers to linearly polarized light having a plane of polarization orthogonal to the S-polarized light L.sub.S. A half wavelength plate 10523 is disposed on the exit side of the total reflection prism 10522, and the S-polarized light L.sub.S emerging from the total reflection prism 10522 is transmitted through the half wavelength plate 10523, whereby it has its plane of polarization rotated by 90.degree. and is converted into P-polarized light L.sub.P *. Also, wedge type lenses 10524 and 10525 for changing the optical path are disposed on the exit sides of the polarizing beam splitter 10521 and the half wavelength plate 10523, respectively, whereby the P-polarized light L.sub.P transmitted through the polarizing beam splitter 10521 and the P-polarized light L.sub.P * converted by the half wavelength plate 10523 have their optical paths changed and intersect each other at a point P.sub.0 on the entrance side surface of a liquid crystal light valve 10527 and become combined light.
Accordingly, in this projector, the liquid crystal light valve 10527 can be illuminated by both of the S-polarized light L.sub.S and the P-polarized light L.sub.P separated by the polarizing beam splitter 10521 and therefore, the utilization efficiency of the light can be made better than in the projectors shown in FIGS. 1 and 2.
However, in the projector shown in FIG. 3, that surface of the polarizing beam splitter 10521 which is in contact with the total reflection prism 10522 and the surface thereof opposed to that contact surface have nothing to do with the optical path of the non-polarized parallel light entering the polarizing beam splitter 10521 and therefore are made. into rough surfaces. On the other hand, where the light source 10511 has a finite size, the non-polarized parallel light emerging from the condenser lens 10514 does not become completely parallel light and enters the polarizing beam splitter 10521 with a certain angle of expanse. As a result, the non-polarized parallel light is incident on said contact surface of the polarizing beam splitter 10521 and the surface thereof opposed to said contact surface and is scattered thereby before and after it is incident on the acting surface 10521a of the polarizing beam splitter 10521, and this leads to a problem that the quantity of light is lost.
Also, to prevent such loss of the quantity of light, there is a problem that the size of the polarizing beam splitter 10521 must be made considerably large relative to the effective irradiation area of the non-polarized parallel light emerging from the condenser lens 10514.
It is a first object of the present invention to provide a polarizing illuminating device which can prevent any loss of the quantity of light and a projector provided with such device.
In a projector using a light modulating element such as liquid crystal, as previously described, image forming means such as a liquid crystal device (LCD) is illuminated by illuminating light provided by an illuminating optical device comprised of a light source, a reflector, etc., and the image thereof is enlargedly projected onto a screen by a projection lens. As a method of improving the utilization efficiency of radiation emitted from the light source, there has been proposed a method as shown, for example, in FIGS. 4 and 5 of the accompanying drawings wherein unutilized light which does not reach the image forming means is imaged near the light source and is reutilized as a secondary light source.
In the method shown in FIG. 4, radiation emitted from a light source 10601 is converted into substantially parallel light (hereinafter referred to as the "parallel light") by a parabolic reflector 10602. A plane mirror 10604 is provided around a liquid crystal device 10603, and of the parallel light emerging from the parabolic reflector 10602, parallel light L.sub.2G directly incident on the plane mirror 10604 is reflected toward the parabolic reflector 10602 by the plane mirror 10604 and forms the image of the light source 10601 near the focus of the parabolic reflector 10602. With this image as a secondary light source, the parallel light L.sub.2G is again converted into parallel light L.sub.2G ' by the parabolic reflector 10602, whereby it becomes illuminating light for the liquid crystal device 10603. The light source 10601 is provided near the focus of the parabolic reflector 10602, and the radiation emitted from the light source 10601 is reflected by the parabolic reflector 10602 to thereby provide parallel lights L.sub.1G and L.sub.2G.
The reasons why besides the parallel light L.sub.1G directly illuminating the liquid crystal device 10603, the parallel light L.sub.2G illuminating the other portion than the liquid crystal device 10603 exists in the parallel light emerging from the parabolic reflector 10602 are:
(1) that the parabolic reflector 10602 usually has a rotation-symmetrical shape and therefore the cross-section of the illuminating light emerging from the parabolic reflector 10602 is circular, whereas the shape of the liquid crystal device 10603 is rectangular; and
(2) that the vicinity of the center of the illuminating light assumes higher luminance than the marginal portion thereof and therefore it is more advantageous for the heightening of the luminance of the image enlargedly projected onto a screen to provide the liquid crystal device 10603 (illuminated member) in the central portion of the illuminating light.
As previously described, the liquid crystal device usually has a polarizer and a polarizing plate called an analyzer forwardly and rearwardly thereof, and in a liquid crystal projector, more than half of illuminating light is absorbed by the polarizer and lost, and a polarizing illuminating device improved in this point is shown in FIG. 5.
Radiation emitted from a light source 10611 is converted into substantially parallel light by a parabolic reflector 10612. Of this parallel light, P-polarized light L.sub.P is transmitted through a polarizing beam splitter 10613 and S-polarized light L.sub.S is downwardly reflected at a right angle by the polarizing beam splitter 10613 as viewed in FIG. 5, whereby the parallel light is separated into P-polarized light L.sub.P and S-polarized light L.sub.S by the polarizing beam splitter 10613. Here, the P-polarized light L.sub.P refers to linearly polarized light perpendicular to the direction of travel and parallel to the plane of the drawing sheet of FIG. 5, and the S-polarized light refers to linearly polarized light perpendicular to both of the direction of travel and the plane of the drawing sheet of FIG. 5. The P-polarized light L.sub.P transmitted through the polarizing beam splitter 10613 is utilized as polarized illuminating light for a liquid crystal device (not shown) provided at the left of the polarizing beam splitter 10613 as viewed in FIG. 5. On the other hand, the S-polarized light L.sub.S reflected by the polarizing beam splitter 10613 is upwardly reflected as viewed in FIG. 5 by a plane mirror 10614 provided below the polarizing beam splitter 10613, whereafter it again enters the polarizing beam splitter 10613 and follows an optical path opposite to the forward route and returns to the vicinity of the focus of the parabolic reflector 10612. As a result, the image of the light source 10611 is formed near the focus of the parabolic reflector 10612, and with this image as a secondary light source, the S-polarized light L.sub.S is again converted into parallel light L.sub.G ' by the parabolic reflector 10612 and enters a polarizing beam splitter 10613. At this time, the S-polarized light L.sub.S has its polarization disturbed by the light source 10611 and reflector 10612 and therefore, the parallel light L.sub.G ' converted by the parabolic reflector 10612 becomes light including P-polarized light and behaves similarly to the light emitted from the aforementioned light source 10611. Assuming that by the above-described operation being repeated, there is no loss by the reflection on the parabolic refector 10612 and plane mirror 10614, the radiation emitted from the light source 10611 can all be utilized as polarized illuminating light for the liquid crystal device.
However, the above-described method of improving the utilization efficiency of the radiation emitted from the light source suffers from a problem as shown below.
As shown in FIG. 6 of the accompanying drawings, the light source 10621 actually has a finite size and therefore, the light emerging from the parabolic reflector 10622 does not become completely parallel light. Assuming that the distance from the focus F of the parabolic reflector 10622 to any reflection point K on the parabolic reflector 10622 is D and the diameter of the light source 10621 is 2r, the light reflected at the reflection point K is cone-shaped light of .+-..theta..congruent.tan.sup.-1 (r/D). As typical ones of such light, there are light L.sub.C1 and light L.sub.C2 as shown in FIG. 6. The light L.sub.C1 has an angle of .theta..sub.1 with respect to the central ray and therefore, the angle of incidence and the angle of reflection at a reflection point K.sub.1 and both .theta..sub.1. Accordingly, even when the light L.sub.C1 emerges from the parabolic reflector 10622 and is reflected by the plane mirror 10623 and thereafter is returned to the parabolic reflector 10622, the light L.sub.C1 has an angle of .theta..sub.1 with respect to a line linking a reflection point K.sub.1 ' on the parabolic reflector 10622 and the focus F of the parabolic reflector 10622 together and therefore, the light L.sub.C1 is again returned to a place considerably far from the light source 10621 with a result that the size of the secondary light source becomes large. When the size of the secondary light source becomes large, the angle .theta. represented by .+-..theta..congruent.tan.sup.-1 (r/D) also becomes large and therefore, the light is eclipsed by the pupil of a projection lens, not shown, and the efficient utilization of the radiation emitted from the light source 10621 cannot be attained. Also, the other light L.sub.C2 becomes lost light because it is not reflected toward the parabolic reflector 10622 by the plane mirror 10623. This is because the light L.sub.C2 has an angle .theta..sub.2 with respect to the central ray and therefore is incident on the plane mirror 10623 at an angle of incidence .theta..sub.2 and thereafter is reflected at an angle of reflection .theta..sub.2.
It is a second object of the present invention to provide an illuminating optical device and a projector in which the utilization efficiency of light can be improved.